With the incessant development and progress of the mobile communications system, the qualities of services provided by the mobile communications system become increasingly higher. In order to maintain long-term competition advantages of 3GPP (the 3rd Generation Partnership Project), the task of stipulating standards for LTE-A (Long Term Evolution-Advanced) is currently in progress. To further enhance spectral efficiency and user throughput of the communications system, the CA (Carrier Aggregation) technology has been introduced into LTE-A. The UE (User Equipment) in the carrier aggregation technology can make simultaneous use of multiple component carriers (CCs) (each corresponding to one cell) to perform uplink and downlink communication, whereby high-speed data transmission is supported. In these aggregated cells, one of them is a primary cell, and the remaining ones are secondary cells.
Carrier aggregation of the LTE system can be substantially divided into cell aggregation inside a base station, and cell aggregation between base stations, etc. Cell aggregation inside a base station means that service cells aggregated by the UE all belong to the same base station. In order to protect transmission security of the air interface, data and signaling should be encrypted, while security keys are not transmitted at the air interface but are derived at the UE and the base station, respectively. With respect to cell aggregation inside a base station, the same security key is used regardless at which cell the data is transmitted. The security key should be derived every time a handover occurs, and the valuation of NCC (Next Hop Chaining Counter) used for derivation of the security key is determined by MME (Mobility Management Entity), and sent to the user equipment (UE) to enable the user equipment to determine the specific mode of deriving the security key according to the valuation of NCC. With the continued development of communications technology, carrier aggregation between base stations is also a direction of future consideration, and security problem concerned therewith is also an aspect of interest to the professionals.